In the metallurgical industry, pulverized coal is generally injected as combustible into blast furnaces. It is important, in order to ensure good functioning of the blast furnace, that the pulverized coal is of good quality, i.e. that the pulverized coal has the right particle size distribution and the lowest humidity level. The pulverized coal is generally produced in a grinding and drying installation, wherein raw coal is ground, dried and classified before the resulting pulverized coal is fed to a storage bin before being injected into a blast furnace. The water contained in the freshly ground coal is spontaneously evaporated with carrying out the mixture by a stream of hot drying gas from the pulverizer into a filter. Inside the filter, the pulverized coal is separated from the drying gas stream and fed to the storage bin. Part of the drying gas is recirculated and mixed with combustion waste gas inside a hot gas generator before it is reintroduced into the pulverizer. The remaining portion of the drying gas is evacuated via an evacuation line leading to a stack.
The recirculation of drying gas allows operating the circuit on a self-inert basis and having a suitable temperature of the drying gas at the pulverizer gas inlet of about 300° C. The recirculation of drying gas however also has its drawbacks; these are a high dew point of the drying gas and high fuel gas consumption for the combustion inside the hot gas generator.
It has been suggested to use waste gas from a hot stove plant as drying gas in the grinding and drying installation. Waste gas exiting such a hot stove plant generally has a temperature of about 100 to 350° C., depending on the operation of the hot stove plant. This hot stove waste gas is sucked by a fan at the stove off-gas collector and conveyed to the grinding and drying installation. More particularly, the hot stove waste gas is fed into the recirculation line and mixed with recirculation drying gas and combustion waste gas of the hot gas generator. The hot stove waste gas is added at a fixed flow rate to the circuit. The fixed flow rate of hot stove waste gas has to be sufficiently low, such that a certain flow rate of recirculation drying gas is always used. This is indeed necessary to be able to control the pressure of the drying gas at the pulverizer gas inlet. Also, the composition of hot stove waste gas may present undesired peaks of CO and O2. It must therefore be possible to shut-off the supply of hot stove waste gas and switch to a conventional operating mode wherein only recirculation drying gas is mixed with the combustion waste gas inside the hot gas generator. In order to regulate the pressure of the drying gas at the pulverizer gas inlet, a control damper in the evacuation line is operated in such a way as to adjust the pressure distribution inside the grinding and drying circuit.
The main disadvantage of this system is that the hot stove waste gas is added to the circuit at a fixed flow rate and that, therefore, hot stove waste gas cannot be used in a more energetically efficient way.